As an overheat detection circuit mounted on the integrated circuit, those circuits shown in the drawing FIG. 4A through FIG. 4C have been known. In FIG. 4A, the circuit utilizes a thermal characteristic of diode's forward voltage drop, which detects the temperature of a place where the diode is located, and the said circuit produces an overheat detective signal when the temperature reaches a certain degree.
In FIG. 4B, an output is produced by the overheat detective signal in accordance with the increase of the leakage current of the base opened transistor, i.e. the increase of the backward current of the parasite diode between the base and the emitter indicated by dotted line.
Furthermore, in FIG. 4C, the overheat detection circuit senses the temperature by using the thermal characteristic of the constant-current source 1 and bipolar transistor 3 through the base resistance 2, and at the same time amplifies the detection current.
For that reason, this circuit is designed to flow the output current of the constant-current source 1 from the joint of the base 3 of the transistor 3 and the base resistance 2 to the base resistance 2.
And the collector current of the bipolar transistor 3 is transformed into the voltage signal by the pull-up resistance 4 and this voltage signal goes through the buffer 5. Thus the overheat detection current might be produced. The base resistor 2 is usually formed by a semiconductor pattern and so forth, surrounded by the well region 2a which is clamped to the base 3a of the transistor 3 in order to be isolated from the other circuit elements.
However, the above-said known overheat detection circuits used for the semiconductor device has advantages and disadvantages at the same time. For example, both the circuit shown in FIG. 4A and the circuit shown in FIG. 4C have an advantage and a disadvantage. It is easy to define the temperature freely to produce the detective signal, but it is difficult to match the transition into the overheat condition to what temperature of the detection.
As to the circuit in FIG. 4B, an advantage, on one hand, to detect correctly the transition into the overheat conditions since the leakage current changes rapidly in accordance with the overheat conditions. On the other hand, it is difficult to design a circuit to obtain the stable operation.
As the circuit in FIG. 4C has a base resistance, it is practical to use the circuit and it has an advantage to utilize the current amplification of the transistor without further additional elements.
In considering the above-said problem, such an overheat detection circuit is requested as can detect correctly the transition into the overheat condition and is easy to utilize. In replying to the request, one idea is to make react to the leakage current as in FIG. 4B, maintaining the advantage of the overheat detection circuit as in FIG. 4C. Therefore, it is the technical problem to improve the detection circuit based on the circuit in FIG. 4C.